A
very thin dividing wall will serve for the purpose, for instance, a film
of oil. But ordinary oil will rub off, so it is better to cover the
surface with an oil-like linseed which oxidizes to a hard elastic and
adhesive coating. If with linseed oil we mix iron oxide or some other
pigment we have a paint that will protect iron perfectly so long as it
is unbroken. But let the paint wear off or crack so that air can get at
the iron, then rust will form and spread underneath the paint on all
sides. The same is true of the porcelain-like enamel with which our
kitchen iron ware is nowadays coated. So long as the enamel holds it is
all right but once it is broken through at any point it begins to scale
off and gets into our food.
Obviously it would be better for some purposes if we could coat our
iron with another and less easily oxidized metal than with such
dissimilar substances as paint or porcelain. Now the nearest relative to
iron is nickel, and a layer of this of any desired thickness may be
easily deposited by electricity upon any surface however irregular.
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